A. Field of the Invention
This invention relates to a digital computer memory system and, more particularly, to a method and apparatus for configuring additional memory used with a stand alone digital computer system including a single board central processing unit and limited on-board memory. The invention provides an efficient memory system configuration adaptable to accommodate and effectively utilize additional memory that is not of a pre-determined size and also that is not physically restricted to be connected in any specific backplane slot.
B. Prior Memory Systems
Previously designed methods of determining the amount of memory hidden behind a memory bus interface utilize a method known as fingerprinting. This method requires physical connections to be reserved on a memory bus which connect each memory board to the memory controller logic. These connections, which are referred to as bus lines, are defined to indicate that a memory board is present, the number of memory boards present and, in encoded form, the density of such memory boards. This requires a fairly large amount of logic in the memory controller of the computer system and also requires the dedicated use of a large number of bus lines. Because of this, in prior art systems changing the amount of memory connected required the services of a skilled technician to set the necessary switches to indicate the amount of memory connected.
One of the functions that the memory controller logic performs is to determine the amount of memory present. This information is encoded on dedicated memory bus lines which are connected between the memory controller logic and each memory board. Thus, the number of physical bus lines dedicated to transmitting this memory density information is directly proportional to the number of memory boards such system supports.
In the prior art, the physical location of memory boards in the system is restricted. The location of the board depends on the density of the memory contained on the board. The higher density boards have to be physically closer to the memory controller than the lower density boards. The essence of the fingerprinting method is that it depends on certain things to be in a certain sequence. Thus, a memory board which contains 1 Mbyte (1,048,576 bytes) of memory space could not be connected to the memory bus in the same physical location that a 2 Mbyte (2,000,000 bytes) or more memory board would be connected.
Much of the memory controller logic is for the starting address offset function. The rest of the logic is required because the information output by the memory boards is "dumb" information, that is, each memory board knows only about itself. The boards do not know if there are other memory boards connected the memory bus. Also, the prior art has no facility for the memory boards to encode data. For every piece of information that each memory board needs to supply the memory controller, there has to be a bus line dedicated for that information.
While the prior art is a very adequate method of configuring and controlling memory, there is a need for advancement. In particular, in a single board computer system most of the intelligence must be placed on one board. Since there are physical limitations in the amount of logic that a single board can support and also limitations in the number of external physical connections such board can support, there is a need to economize both. The prior art does not adequately address these limitations.